fast and beautiful way to convert vector ranges to indexes

Hi there,

my question could easily be solved with a simple loop but I'm curious if there is a nice genuine matlab way of doing this: I have a vector "v1" containing 10 entries. First 5 belong together, next 2 belong together and the last 3 again (somehow). So I got a vector "v2" with

   v2 = [5; 2; 3];

I want now something like

   v3 = [1 1 1 1 1 2 2 3 3 3];

So I could access my v1 vector with:

   v1(v3==1);

(Background for the question are different colors for each group with the plot-command.)

Thanks already in advance - I'm sure Matlab holds a nice and short way of doing this :-)

Vincent

Answer 1

[Matt J]

c=cumsum(v2);
v3=zeros(1,c(end));
 v3([1,c(1:end-1)])=1;

v3=cumsum(v3);
 v3(end)=[],

[Vincent]

Nice, except for one little mistake (line 3, move the first "1" out of the square brackets). Also the last line didn't seem to be necessary as we took "end-1" beforehand I guess…

Unfortunately not a single line of code, but I guess I've been looking for something like "cumsum". Still odd, that there's no more condensed way in Matlab to achieve this…

[Matt J]

Here's a 2-line version

 v3(cumsum(v2)+1)=1;
 v3=cumsum(v3(1:end-1))+1

but the number of lines is inconsequential anyway. You can always reduce code to 1 line by wrapping it in your own mfile.

Answer 2

[Andrei Bobrov]

v2 = [5; 2; 3];

ii = cumsum(v2);
v3 = zeros(ii(end),1);
v3( ii - v2 + 1) = 1;
v3 = cumsum(v3);

[Vincent]

this seems pretty similar to Matt J's solution to me, right?

[Andrei Bobrov]

of course

Answer 3

[Azzi Abdelmalek]

v2 = [5; 2; 3];
v3=cell2mat(arrayfun(@(x) repmat(x,1,v2(x)),1:numel(v2),'un',0))

[Matt J]

not really loopless...

[Vincent]

this is at least a single-line solution. But Matt is right - thanks for your answer anyway!

Answer 4

[Matt J]

Here's a 1-liner,

 [~,v3]=histc(1:sum(v2),[1;cumsum(v2(:))+1])

[Matt J]

Although, to avoid summing v2 twice, it is best to break it into 2 lines

 c=cumsum(v2(:));
 [~,v3]=histc(1:c(end),[1;c+1])

[Vincent]

Oh I would never have thought about histc! Nice idea! Thanks

Answer 5

[Jan]

See http://www.mathworks.de/matlabcentral/fileexchange/41813-runlength:

v2 = [5; 2; 3];
v3 = RunLength(1:length(v2), v2);

[Vincent]

I just looked up your function - looks pretty nice! Unfortunately I have to stick on built-in functions in my case :-/